Integrated devices are usually produced on substrates in the form of wafers, which mainly serve as a medium for their manufacture. However, the rise in the level of integration and in the performances expected of these devices results in an ever-increasing linkage between their performances and the characteristics of the substrate on which they are formed. This is particularly the case of radiofrequency (RF) devices processing signals with a frequency of between approximately 3 kHz and 300 GHz, which are more particularly applied in the telecommunications field (cellular telephony, Wi-Fi, BLUETOOTH®, etc.).
By way of an example of device/substrate coupling, the electromagnetic fields, originating from the high-frequency signals propagating through the devices, penetrate deeply into the substrate where they interact with any charge carriers located there. This results in problems of non-linear distortion of the signal (harmonics), unnecessary consumption of a portion of the energy of the signal by insertion loss and possible influences among components.
Hence, RF devices display characteristics governed both by their architectures and production processes and by the ability of the substrate on which they are manufactured to limit insertion losses, diaphonies between neighboring devices and phenomena of non-linear distortion-generating harmonics.
With the explosion of the demand for data, created by “multimedia” applications, changes in mobile telephony standards (2G, 3G, LTE, LTE-A, etc.) likewise impose increasingly stringent specifications on RF components. The RF performances of these components typically need to be guaranteed between −20° C. and +120° C., implying that the electrical properties of the substrate are stable within this temperature range.
Radiofrequency devices, such as switches and antenna adapters in addition to power amplifiers, can be produced on different types of substrate.
Silicon-on-sapphire substrates, commonly referred to as SOS, are known in which the components, produced using microelectronic technologies in the superficial layer of silicon, benefit from the insulating properties of the sapphire substrate, which are temperature-independent. For example, antenna switches and power amplifiers manufactured on this type of substrate display very good merit factors, but are mainly used for niche applications owing to the overall cost of the solution.
Substrates based on high-resistivity silicon are also known, comprising a supporting substrate, a trapping layer arranged in the supporting substrate, a dielectric layer arranged on the trapping layer and a semiconducting layer arranged on the dielectric layer. The supporting substrate usually has a resistivity greater than 1,000 ohm·cm. The trapping layer may comprise undoped polycrystalline silicon. The combination of a high-resistivity supporting substrate and a trapping layer according to the state of the art makes it possible to reduce the device/substrate coupling mentioned above, thereby ensuring good performances of the RF devices. In this respect, the persons skilled in the art will find a review of the performances of RF devices manufactured on the high-resistivity semiconducting substrate known in the prior art in “Silicon-on-insulator (SOI) Technology, manufacture and applications,” points 10.7 and 10.8, by Oleg Kononchuk and Bich-Yen Nguyen, from Woodhead Publishing.
Nevertheless, these substrates fail to fulfil the most stringent specifications; for example, when localized heating beyond approximately 80° C. occurs, the resistivity of these substrates declines due to generation of heat carriers in the substrate and device/substrate coupling once again becomes a major contributor to attenuation and distortion of the signal and interference among components. Deteriorations in performance are also observed when the temperature falls to below 0° C.